1. Field of the invention
The invention relates to ring interferometers and more particularly to a method for detecting propagation time variations in such an interferometer, to the device for implementing this method and its application thereof to controlling the mean wave length of the source.
2. Description of the Prior Art
In a ring interferometer, two contrarotating waves are propagated in a ring wave guide, for example an optical fiber coil; after propagation the two waves are in perfect phase at the ends of the coil in the absence of non reciprocal effects, particularly in the absence of SAGNAC effect due to rotation of the interferometer.
On the other hand, when the interferometer is rotated with respect to a Galilean reference, the phase-shift between the two waves emerging from both ends of the coil is given by ##EQU1## where L is the length of the coil, R its radius, .lambda. the wave length, c the speed of light in a vacuum and .omega. the speed of rotation.
In the absence of other non reciprocal effects, i.e. in the absence of magnetic field and so of Farraday effect, and in the absence of displacement of material and so of the FIZEAU effect, measurement of the phase-shift leads to measurement of the speed of rotation, provided that all the other parameters are stable, particularly the wave length of the source.
Now this wave length varies, in particular as a function of the temperature. For a source formed by a super luminescent diode, or "DSL", on gallium arsenide, variation of the wave length of the source is of the order of 4.times.10.sup.-4 per degree Celsius. This variation leads to a corresponding variation of the propagation time of light in the wave-guide because of the dispersion.
Taking into account the precision now obtained in the measurement of speed using an optical fiber ring interferometer with a DSL as source and a monomode optical fiber with polarization conservation, these wave length variations affect the accuracy of the measurement. Consequently, it is necessary to evaluate them, either for correcting them or for taking them into account in the measurement of the phase-shift.